1. Field of the Invention
The present invention relates to a novel polypropylene resin composition, a gas permeable film comprising the same and provided with satisfactory optical characteristics (transparency, gloss and see throughness) as well as an excellent gas permeability (water vapor, oxygen and ethylene gas) without damaging a rigidity and a method for maintaining a freshness of vegetables and fruits over a long period of time using the above gas permeable film for a wrapping film.
2. Description of the Related Art
A polypropylene film has a rigidity and excellent optical characteristics (transparency, gloss and the like) which are required to packaging materials and therefore has widely been used. However, when a polypropylene film is used as a wrapping material for vegetables and fruits, it does not have a satisfactory gas permeability against water vapor, oxygen and ethylene gas, and therefore it has been used making holes for permeating gas. In this case, ethylene gas generated from vegetables and fruits which are wrapped matters can sufficiently be discharged, and oxygen necessary for breathing of vegetables and fruits can sufficiently be taken in. However, water vapor can not be prevented from vaporizing from vegetables and fruits, so that there has been the problem that vegetables and fruits are liable to be dried and freshness thereof is lost.
Known is a film which shows a gas permeability without carrying out such drilling work and can maintain a freshness of vegetables and fruits (International Publication WO99/51665), but there has been the problem that the film does not have satisfactory transparency, gloss and see throughness (hereinafter referred to as ┌optical characteristics┘) which are required in an actual distribution route.
An object of the present invention is to provide a resin composition capable of preparing a gas permeable film which is endowed with satisfactory optical characteristics as well as an excellent gas permeability (water vapor, oxygen and ethylene gas) without damaging a rigidity and a method for maintaining a freshness of vegetables and fruits over a long period of time by using the above gas permeable film for a wrapping film.
Intensive researches repeated by the present inventors in order to achieve the object described above have resulted in finding that a film produced from a polypropylene resin composition comprising a propylene polymer (A) and an ethylene/propylene/butene random copolymer (B) containing the specific amounts of butene and ethylene has an excellent gas permeability (water vapor, oxygen and ethylene gas) without damaging a rigidity and is endowed with satisfactory optical characteristics in combination therewith. Thus, they have come to complete the present invention.
That is, the polypropylene resin composition according to the present invention is characterized by comprising a propylene polymer as a component (A) and an ethylene/propylene/butene random copolymer as a component (B). More specifically, it is characterized by comprising 40 to 95% by weight of a propylene polymer as the component (A) and 5 to 60% by weight of an ethylene/propylene/butene random copolymer having a butene content of 3 to 40% by weight and an ethylene content of 10 to 30% by weight as the component (B).
Further, the polypropylene resin composition according to the present invention is characterized by that a ratio xcex7B/xcex7A of an intrinsic viscosity xcex7B of the component (B) described above to an intrinsic viscosity xcex7A of the component (A) described above falls in a range of 0.5 to 1.3. Also, the polypropylene resin composition according to the present invention is characterized by that the component (B) described above has an intrinsic viscosity xcex7B falling in a range of 0.5 to 3.0 dl/g.
Further, the gas permeable film according to the present invention is characterized by that it is a single layer or multilayer film comprising at least one layer of a film of the polypropylene resin composition described above according to the present invention and having a thickness of 10 to 100 xcexcm and that it has the following characteristics:
(1) a permeability TH2O of water vapor is 9 to 50 g/m2xc2x724 h (based on JIS Z 0208),
(2) a permeability TO2 of oxygen is 600 to 12500 nmol/m2xc2x7sxc2x7100 kPa (based on JIS K 7126-A) and
(3) a permeability Tethylene of ethylene gas is 600 to 22500 nmol/m2xc2x7sxc2x7100 kPa (based on JIS K 7126-A).
Further, the gas permeable film according to the present invention is characterized by that the film described above is oriented at least in a monoaxial direction and includes a film oriented at an orienting magnification of 3 to 60 times at least in a monoaxial direction.
The vegetable and fruit freshness long term-maintaining method according to the present invention is characterized by using the gas permeable film according to the present invention as a film for wrapping vegetables and fruits.
Shown is a flow sheet of a continuous polymerization apparatus used in the examples and the comparative examples.